In my earlier U.S. Pat. No. 4,238,705 there is disclosed an aluminosilicate glass composition useful in high temperature lamps and consisting essentially of oxides in approximate percent by weight 54-71 SiO.sub.2, 12-18 Al.sub.2 O.sub.3, 4-8 CaO, 10-23 BaO, and 0-3 R.sub.2 O wherein R is an alkali metal ion, along with minor amounts of incidental impurities, residual fluxes, and refining agents. The aggregate SiO.sub.2 and Al.sub.2 O.sub.3 content in said prior art glass lies within the approximate range 73-83 weight percent, the aggregate BaO and CaO content lies within the approximate range 17-30 weight percent and the weight ratio between BaO and CaO lies within the approximate range 2.3-3.5 to provide improved reboil resistance. The average thermal co-efficient of expansion for said prior art glass in the 0.degree.-300.degree. C. temperature range lies between about 37.times.10.sup.-7 cm./cm./.degree.C. to about 50.times.10.sup.-7 cm./cm./.degree.C. for direct sealing to the lamp glass envelope of the lamp inleads which can be either tungsten or molybdenum metal. The regenerative halogen cycle lamp described in this patent further discloses a utilization of larger diameter inleads being joined to said tungsten or molybdenum lamp inleads in the press seal region as a means for improving the mechanical support of said lamp.
It is now been discovered that still further improvements for said type incandescent lamp construction can be provided with only a relatively minor modification being made to the glass composition. More particularly, it has been found that relatively minor amounts of ZrO.sub.2 addition in said glass composition produces a significant reduction in the seed count of the molten glass when refined and thereafter drawn into tubing thereby improving the mechanical strength and optical clarity of the lamp glass envelope using said modified glass composition. This effect was unexpected since ZrO.sub.2 has not been recognized previously as a glass refining agent. An addition of ZrO.sub.2 in minor amounts to this aluminosilicate glass further raises the strain point desirably without unduly causing its devitrification during the glass forming procedure. The strain point elevation is desirable since the glass envelope in a high temperature lamp can develop permanent stresses on heating and cooling during on and off cycles of the lamp operation. Since these lamps are usually under pressure of several atmospheres when operated, a higher strain point in said lamp glass envelope lessens its prospects for mechanical failure due to any permanent stress condition developed when the lamp is operated.
Accordingly, the present invention relates to an improved seal glass composition which has been found particularly useful to provide the hermetic seal means for an incandescent lamp being operated at elevated temperatures often exceeding 500.degree. C. and higher. Said type incandescent lamp designs require a transparent envelope material capable of withstanding these elevated temperatures and wherein said envelope is also sealed hermetically to the electrical inleads. Among these lamp types is a regenerative cycle halogen containing lamp wherein a resistive incandescent filament providing the light source in said lamp is joined at each end to an inlead and with both of said inleads being fusion sealed directly to the lamp glass envelope. Lamps having said structural configuration are now widely being employed in a variety of lighting applications including projector lamps, automotive headlamps and even household illumination sources. In said product applications, the regenerative halogen cycle lamp can be mounted in an outer reflector member but can also simply be contained within an outer glass envelope.